Here the scattering dyadic \(S̭ ({r},{r}^{\prime })\)  contains a non-standard scaling factor \(-Jk/ (4 \pi )\) ,  [1]}, for the sake of notational and dimensional simplicity in later sections. Note that the case of \({r} = {r}^{\prime }\)  corresponds to forward scattering and that reciprocal systems exhibit the symmetry property
\(S̭({r},{r}^{\prime }) = S̭^(-{r}^{\prime },-{r}).\) 
